1. Field of the Invention
The present invention relates to an apparatus which reproduces signals from an optical disk such as a DVD-ROM, and in particular to an apparatus which detects a frame number in the reproduced signals.
2. Description of Prior Art
Recently, an optical disk has been developed to record and reproduce information optically with a laser light, and various formats are proposed to record information in an optical disk. Synchronizing signals are necessary in any format. When an optical disk is reproduced, the synchronizing signals are reproduced and counted, and a frame is decided according to the count value. That is, a frame is detected by counting the synchronizing signals. However, when a disk is rotated anomalously, the time length for a frame becomes longer or shorter. Then, synchronizing signals is counted erroneously, and it becomes uncertain which frame in a sector is reproduced. Thus, the reproduced signals are all erroneous.
Japanese Patent laid open Publication 8-289249/1996 describes an apparatus and a method for solving this problem. A recording medium or DVD-ROM has digital data in the DVD-ROM format wherein a sector consists of lines, each line consisting of frames of odd numbers and those of even numbers. A frame consists of synchronizing signals (of sync numbers) and data. In the DVD-ROM format, a synchronizing signal has one of predetermined patterns, and when a DVD-ROM is reproduced, a frame number is detected according to a combination of at least two, or preferably consecutive three or four synchronizing signals for the sake of stability of the frame number detection.
However, in the DVD-ROM format, because only a 13-bit pattern is varied in the synchronizing signals, a distance between two synchronizing signals becomes very short. If the bit length of the pattern of synchronizing signal is short, possibility of erroneous detection of frame numbers increases. On the other hand, if the bit length of the pattern in the synchronizing signal is increased, data efficiency becomes worse. If the number of synchronizing signals necessary for the determination of frame number is increased, the probability of erroneous detection is decreased. However, no errors cannot be allowed, and a ratio for the correct detection of frame number is liable to be decreased. Therefore, it is desirable to detect a frame number correctly in the data including synchronization patterns of short code distances between them.